Cambridge A-Level Biology | Allergies & Hypersensitivity: Mechanism, IgE Antibodies, and Immune Response

Master the Cambridge A-Level Biology | Allergies & Hypersensitivity: Mechanism, IgE Antibodies, and Immune Response

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Before diving into Cambridge A-Level Biology | Allergies & Hypersensitivity: Mechanism, IgE Antibodies, and Immune Response ensure you have gone through our previous guide : Monoclonal Antibodies & Interferons: Production, Uses & Role in Infection & Cancer | A-Level Biology

Table of Content

Introduction to Hypersensitivity & Allergies
What are Allergens? (Common Triggers & Characteristics)
The Role of IgE Antibodies in Allergic Response
Step-by-Step Mechanism of Type I Hypersensitivity

Sensitisation Phase (Primary Exposure
Effector Phase & Degranulation (Secondary Exposure)

Chemical Mediators: Histamine & Its Physiological Effects
Clinical Manifestations: Localized vs Systemic Reactions (Anaphylaxis)
Summary Table: Normal Immune Response vs Allergic Response
AO1 Knowledge with Understanding (Direct & Recall Questions)
AO2 Application of Knowledge (Diagram & Labeling Questions)
AO3 Experimental Skills & Data Interpretation (Graph & Table Questions)

Introduction to Hypersensitivity & Allergies

In a healthy immune response, the immune system distinguishes between self and non-self antigens, targeting only harmful foreign pathogens (like bacteria, viruses, and parasites).

However, in some individuals, the immune system exhibits an exaggerated or inappropriate immune response to harmless environmental substances. This hyper-reactive state is known as Hypersensitivity.

💡 Key Definition:

📝Hypersensitivity refers to an altered, inappropriate, or oversensitive immune response by the host's immune system against non-pathogenic foreign antigens (allergens) or self-antigens, leading to tissue damage, inflammation, or disease.

Classification of Hypersensitivity (Coombs & Gell System)

Hypersensitivity is broadly classified into four main types (Types I to IV).
In A-Level Biology, our primary focus is on Type I Hypersensitivity, commonly known as an Allergy.
Type I is Hypersensitivity or Allergy Which is Mediated by IgE antibodies and mast cell degranulation within minutes of exposure For example : Asthma, Hay Fever, Anaphylaxis.
Types II–IV Involve IgG/IgM antibodies or T-cell mediated delayed responses For Example : Blood transfusion reactions, Contact dermatitis).

💡Related study to understand about the Our Immune System: Cellular Component Framework & Self vs Non-Self Recognition | Cambridge A-Level Biology Core Notes

What are Allergens? (Common Triggers & Characteristics)

An allergen is a specific type of non-pathogenic antigen normally harmless substance that induces an allergic or Type I hypersensitive response in susceptible individuals (atopic individuals).

💡Important Distinction
📝All allergens are antigens (because they stimulate an immune response), but not all antigens are allergens (since normal antigens originate from disease-causing pathogens).

Key Characteristics of Allergens

For an environmental molecule to act as an allergen, it typically possesses specific biochemical properties:
Allergens are typically small proteins or glycoproteins with low molecular weight that can easily diffuse through mucosal membranes (e.g., respiratory or digestive lining).
They have high solubility and can dissolve in body fluids (mucus or saliva), allowing rapid absorption upon contact.
Many prominent allergens possess protease enzyme activity, which actively breaks down epithelial tight junctions, facilitating their penetration into tissues.
They are often stable molecules resistant to desiccation, heat, or mild enzymatic degradation.

Common Environmental Triggers :

Allergens enter the body through four primary routes:

Route of Exposure	Trigger / Allergen Source	Resulting Clinical Condition
Inhalation (Airborne)	Plant pollen grains, House dust mite feces, Fungal spores, Animal dander	Allergic Rhinitis (Hay Fever), Extrinsic Asthma
Ingestion (Food)	Peanuts, Tree nuts, Shellfish, Eggs, Cow's milk	Gastrointestinal distress, Urticaria (Hives), Systemic Anaphylaxis
Injected / Parenteral	Hymenoptera venom (Bee/Wasp stings), Intravenous drugs (e.g., Penicillin)	Acute Systemic Anaphylaxis
Direct Contact (Skin)	Latex, Certain cosmetics, Chemical dyes	Contact Dermatitis, Localized Hives

Step-by-Step Mechanism of Type I Hypersensitivity

The mechanism of Type I hypersensitivity occurs in two distinct phases:
Sensitisation Phase (Primary Exposure): The initial, asymptomatic exposure that primes the immune system.
Effector Phase & Degranulation (Secondary Exposure): The rapid, symptom-producing reaction upon subsequent exposure to the same allergen.

Sensitisation Phase (Primary / First Exposure)

During the first exposure to an allergen, no clinical symptoms are produced.
Instead, the body prepares its specific immune response against the allergen.

Allergen Enters Body

↓

Uptake by Dendritic Cells / APCs

↓

Presentation to Naïve T-Helper Cells → TH2 Cells

↓

IL-4 & IL-13 Secretion → B-Cell Class Switching to IgE

↓

Plasma Cells Produce Secretory IgE Antibodies]

↓

IgE Binds to Fc Receptors on Mast Cells & Basophils

↓

Individual is now SENSITISED

Step 1: Antigen Processing & Presentation

The allergen penetrates mucosal barriers (e.g., respiratory tract) and is engulfed by Antigen-Presenting Cells (APCs), such as mucosal dendritic cells.
The APC processes the allergen and presents its peptide fragments on MHC Class II molecules.

The Two-Step Mechanism of Type I Hypersensitivity (Sensitisation vs. Effector Phase).

Step 2: T-Helper Cell Differentiation (TH2 Response)

APCs present the allergen fragment to naive T-helper (TH) cells. Under the influence of local cytokines, these differentiate predominantly into TH2 cells (Type 2 Helper T cells) rather than TH1 cells.

Step 3: B-Cell Activation & Class Switching

Activated TH2 cells interact with B-lymphocytes and secrete specific interleukins:
Interleukin-4 (IL-4) & Interleukin-13 (IL-13): Trigger B-lymphocytes to undergo heavy-chain class switching, shifting antibody production from (IgM) to (IgE)

Step 4: Secretion of Allergen-Specific IgE Antibodies

Activated B-cells proliferate and differentiate into antibody-secreting plasma cells.
These plasma cells synthesize and secrete large quantities of allergen-specific IgE antibodies into the surrounding tissue fluid and bloodstream.

Step 5: Binding to Mast Cells & Basophils (Sensitisation)

The constant region (Fc region) of the secreted IgE antibodies binds with high affinity to Fc receptors located on the surface membrane of tissue mast cells and blood basophils.

Outcome:

The individual is now sensitised. The mast cells are permanently "armed" with allergen-specific IgE antibodies on their surface, ready to react upon any future exposure.

💡Related study to understand about the Cambridge AS & A Level Biology: Immunity Master Notes (Syllabus 9700)

Effector Phase & Degranulation (Secondary / Subsequent Exposure)

When the sensitised individual encounters the same allergen a second time, the response is immediate (typically occurring within seconds to minutes).

Secondary Allergen Exposure
↓
Cross-linking of adjacent IgE molecules on Mast Cells
↓
Intracellular Signal Transduction & Ca2+ Influx
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Exocytosis of Granules (DEGRANULATION)]
↓
Release of Pre-formed Mediators (Histamine, Proteases)
↓
Synthesis of Newly Formed Mediators (Leukotrienes, Prostaglandins)
↓
(Immediate Hypersensitivity Symptoms: Vasodilation, Bronchoconstriction, Mucus Secretion)

Step 1: Secondary Exposure & Allergen Binding

The allergen enters the body again and diffuses into tissues where sensitised mast cells are present.

Step 2: IgE Cross-Linking

Multivalent allergen molecules bind simultaneously to the antigen-binding sites Fab regions) of at least two adjacent IgE molecules anchored to the mast cell membrane. This process is called cross-linking.

Close-up of IgE Cross-linking on Mast Cell Surface leading to Granule Exocytosis.

Step 3: Intracellular Signaling & Calcium Influx

Cross-linking activates an intracellular signaling cascade, opening calcium channels and causing a rapid influx of calcium ions Ca2+ into the mast cell cytoplasm.

Step 4: Exocytosis (Degranulation)

The rapid increase in cytosolic Ca2+ triggers the fusion of cytoplasmic storage granules with the plasma membrane, releasing pre-stored chemical mediators into the extracellular environment via exocytosis.

Step 5: Immediate Action of Pre-formed Mediators (Early Phase Response)

Within minutes, pre-formed mediators like Histamine bind to specific receptors (H1 receptors) on target tissues and causing -
Vasodilation: Dilates local blood vessels, causing redness (erythema).
Increased Vascular Permeability: Endothelial cells contract, allowing plasma fluid to leak into tissues (causing edema/swelling).
Bronchoconstriction: Smooth muscle of the airways contracts, narrowing the lumen.
Mucus Hypersecretion: Stimulates goblet cells and mucosal glands.

Step 6: Release of Newly Synthesized Mediators (Late Phase Response)

Hours after the initial degranulation, activated mast cells synthesize and secrete secondary inflammatory lipid mediators, primarily Leukotrienes (LTC4) (LTD4) and Prostaglandins (PGD2).

💡Note for A-Level:
📝Leukotrienes cause prolonged and potent bronchospasm (1000 times more potent than histamine) and recruit additional eosinophils and neutrophils to the site of inflammation.

Chemical Mediators: Histamine & Its Physiological Effects

During Type I hypersensitivity, the immediate symptoms experienced by an individual are primarily driven by chemical signaling molecules released during mast cell degranulation. Among these, Histamine is the principal pre-formed mediator.

Synthesis & Storage of Histamine

Histamine is . within mast cells and basophils by the decarboxylation of the amino acid L-histidine (catalyzed by the enzyme histidine decarboxylase).
It is stored inside cytoplasmic secretory granules, bound to negatively charged heparin proteoglycans.
Once released via exocytosis, histamine diffuses rapidly into surrounding tissues and binds to specific H1 histamine receptors (G-protein coupled receptors) on target cell membranes.

Physiological Actions of Histamine

Histamine acts on multiple target organs and tissue types:

Target Tissue / Organ	Mechanism of Action	Physiological Consequence / Symptom
Vascular Smooth Muscle	Stimulates nitric oxide (NO) release, causing relaxation of arterial smooth muscle.	Arteriolar Vasodilation: Leads to localized redness (erythema), increased tissue temperature, and systemic drop in blood pressure.
Vascular Endothelium	Causes endothelial cells to contract and separate, widening intercellular gaps.	Increased Capillary Permeability: Allows plasma proteins and fluid to leak into interstitial spaces, resulting in local tissue swelling (Edema/Wheal formation).
Bronchial Smooth Muscle	Binds H₁ receptors on smooth muscle surrounding bronchioles, inducing sustained contraction.	Bronchoconstriction: Narrows airway diameter, leading to wheezing, chest tightness, and dyspnea (difficulty breathing).
Mucosal Epithelium	Directly stimulates submucosal exocrine glands and goblet cells in the respiratory and GI tracts.	Mucus Hypersecretion: Results in nasal congestion, runny nose (rhinorrhea), and watery eyes.
Sensory Nerve Endings	Stimulates unmyelinated C-fiber nerve endings in the dermis and mucosa.	Pruritus (Itching): Triggers intense itch signals sent to the central nervous system.

Clinical Manifestations: Localized vs Systemic Reactions (Anaphylaxis)

Depending on the route of allergen entry, the concentration of IgE-primed mast cells, and the amount of histamine released, Type I hypersensitivity reactions present as either localized or systemic.

1. Localized Allergic Reactions (Atopic Conditions)

In localized reactions, the allergic response is confined to a specific organ or tissue site where the allergen entered the body.
Allergic Rhinitis (Hay Fever): Airborne allergens (pollens) contact nasal mucosa and cause sneezing, itching, nasal congestion, and clear rhinorrhea.
Allergic Asthma: Inhaled allergens reach lower respiratory airways and causes bronchospasm, mucosal swelling, and excessive thick mucus production, leading to severe airflow obstruction.
Atopic Urticaria (Hives): Skin contact or ingested allergens cause localized dermal mast cell degranulation and raised, red, itchy skin welts .

2. Systemic Reaction: Anaphylaxis (Anaphylactic Shock)

Anaphylaxis is a severe, life-threatening, rapidly developing systemic allergic reaction caused when an allergen (e.g., bee venom, intravenous drugs, peanuts) enters the circulation directly or is absorbed rapidly.

💡Related study to understand about the Cambridge A-Level Biology: Infectious Disease – Malaria (Causes, Transmission & Control)

Pathophysiology of Anaphylactic Shock:

Widespread Vasodilation: Mass histamine release across all blood vessels causes sudden, profound systemic peripheral vasodilation.
Capillary Leakage: Massive fluid loss from the bloodstream into surrounding tissues drastically reduces venous return to the heart.
Hypotension & Collapse: The combined drop in peripheral resistance and cardiac output leads to a sudden plummet in blood pressure (anaphylactic shock), compromising organ perfusion.
Airway Obstruction: Severe edema of the larynx (swelling of the throat/tongue) coupled with severe bronchospasm blocks airflow, risking death by asphyxiation within minutes.

💡Emergency Treatment: Epinephrine (Adrenaline)

📝Anaphylaxis is a medical emergency that requires immediate administration of Epinephrine (Adrenaline) via an intramuscular autoinjector (e.g., EpiPen).

A-Level Mechanism of Action (How Epinephrine Reverses Anaphylaxis)

Stimulates alpha 1-adrenergic receptors: It act as powerful vasoconstriction of blood vessels . Aa a result, it increases peripheral vascular resistance, restores blood pressure, and reduces swelling/edema.
Stimulates beta 2-adrenergic receptors: It Induces potent bronchodilator relaxation of bronchial smooth muscle and opens airways to restore ventilation.
Inhibits Mast Cells: It Suppresses further degranulation and mediator release.

Summary Table: Normal Immune Response vs Allergic Response

Parameter / Feature	Normal Immune Response	Allergic (Type I Hypersensitive) Response
Primary Trigger	Pathogenic foreign substances (bacteria, viruses, toxins).	Normally harmless environmental antigens (Allergens, e.g., pollen, peanuts).
Primary Antibody Class	IgG (in blood/tissue) or IgA (in mucosal secretions).	IgE (bound to tissue mast cells and blood basophils).
Key T-Helper Pathway	T_H1 cell predominant response (activates macrophages & T_C cells).	T_H2 cell predominant response (secretes IL-4, IL-13, IL-5).
Effector Cells Involved	Macrophages, Cytotoxic T-cells (T_C), Neutrophils, B-cells.	Mast cells, Basophils, Eosinophils.
Primary Signal Mechanism	Cytokines activate cellular immunity; complement system destroys targets.	IgE Cross-linking on mast cells → Ca²⁺ influx → Degranulation.
Chemical Mediators	Interferons, Interleukins (IL-2), Chemokines.	Histamine, Leukotrienes (LTC₄/LTD₄), Prostaglandins (PGD₂).
Tissue Outcome	Targeted destruction and clearance of pathogen; tissue repair.	Collateral tissue damage, vasodilation, edema, bronchoconstriction.
Primary Biological Goal	Protection and survival of the host organism.	Unnecessary, exaggerated, and potentially life-threatening hyper-reaction.

Summary & Conclusion

In conclusion, Type I Hypersensitivity (Allergy) represents a misalignment of the body's host defenses where a pathway evolved to protect against parasites is mistakenly activated by harmless environmental proteins (allergens).
The entire process hinges on two critical phases:
Sensitisation (Primary Exposure): Silent priming of the immune system via TH2 signaling, leading to IgE production and its high-affinity binding to tissue mast cells.
Effector Phase (Secondary Exposure): Rapid (IgE} cross-linking triggering calcium influx, mast cell degranulation, and the release of inflammatory mediators like histamine and leukotrienes.
Understanding these cellular mechanisms is pivotal and not only for Cambridge A-Level Biology exams but also for clinical medicine, where targeted therapies like antihistamines and epinephrine (adrenaline) directly counteract these physiological pathways to save lives.

📝AO1 Knowledge with Understanding (Direct & Recall Questions)

Q1: Define the terms Hypersensitivity and Allergen. [2 Marks]

Answer : Hypersensitivity: An exaggerated, inappropriate, or altered immune response by the host's immune system against non-pathogenic foreign antigens or self-antigens, leading to tissue damage or inflammation. [1 Mark]

Allergen: A non-pathogenic, normally harmless substance/antigen that stimulates a Type I hypersensitive (allergic) response in a sensitised individual. [1 Mark]

Q2: State the specific class of antibody and the cell type primarily responsible for mediating Type I hypersensitivity reactions. [2 Marks]

Answer : Antibody Class: Immunoglobulin E (IgE). [1 Mark]

Primary Cell Type: Tissue Mast cells (or blood Basophils). [1 Mark]

Q3: Outline the main events that occur during the Sensitisation Phase of an allergic response. [4 Marks]

Answer : Antigen Presentation: Dendritic cells / APCs engulf, process, and present allergen fragments on MHC Class II molecules to naive T-helper cells. [1 Mark]

TH2 Differentiation & Interleukin Release: T-helper cells differentiate into TH2 cells, which secrete IL-4 and IL-13 [1 Mark]

Class Switching & IgE Secretion: Interleukins induce B-lymphocytes to undergo heavy-chain class switching to become plasma cells that secrete allergen-specific IgE antibodies. [1 Mark]

Binding to Receptors: The Fc region of secreted IgE binds to high-affinity Fc receptors on the surface of tissue mast cells and basophils. [1 Mark]

Q4: Describe the mechanism triggering Mast Cell Degranulation upon secondary exposure to an allergen. [3 Marks]

Answer : IgE Cross-linking: Allergen molecules simultaneously bind to the Fab regions of at least two adjacent IgE antibodies bound to the mast cell membrane. [1 Mark]

Calcium Influx: Cross-linking initiates intracellular signaling that leads to a rapid influx of calcium ions Ca 2+ into the cytoplasm. [1 Mark]

Exocytosis: Increased cytosolic Ca 2+ causes secretory granules to fuse with the plasma membrane, releasing pre-formed inflammatory mediators (like histamine) into the extracellular space via exocytosis. [1 Mark]

Q5: Name two pre-formed mediators and two newly synthesized mediators released during an allergic reaction. [2 Marks]

Answer : Pre-formed Mediators : Histamine, Heparin, or Proteases (Tryptase). (Any two) [1 Mark]

Newly Synthesized Mediators : Leukotrienes Prostaglandins (l or Cytokines

Q6: Explain how Epinephrine (Adrenaline) counteracts the physiological effects of Systemic Anaphylaxis. [3 Marks]

Answer : Vasoconstriction (alpha 1 Receptors Stimulates alpha 1 adrenergic receptors to cause blood vessel constriction, increasing peripheral resistance and restoring blood pressure. [1 Mark]

Bronchodilation ( beta 2 Receptors) Stimulates beta 2 adrenergic receptors to relax bronchial smooth muscle, opening airways to restore breathing. [1 Mark]

Inhibition of Mediators Reduces further mast cell degranulation and capillary permeability, curbing fluid leakage (edema). [1 Mark]

📝AO2 Application of Knowledge (Diagram & Labeling Questions

Context : Study the the following diagram and answer the following questions.

Question 1: (a) Identify the molecules/chemical signals represented by X and Y. (b) State which region of molecule Y binds to the receptor on the Mast cell surface.

Question : 2 (a) Name the specific type of helper T-cell shown in the diagram and state one cytokine it releases to activate the B-lymphocyte (B-L).

(b) Describe the transformation that B-L undergoes to become a Plasma cell.

Answer 1: (a) label X: Cytokines (specifically IL-4 / IL-13) released by TH2 cells. [1 Mark] label Y: IgE antibodies (Immunoglobulin E). [1 Mark]

(b) The Fc region (constant region) of the IgE antibody binds to Fc RI receptors on the mast cell membrane. [1 Mark]

Answer 2: (a) TH2 (Type 2 Helper T-cell). It releases Interleukin-4 (IL-4) or Interleukin-13 (IL-13). [1 Mark]

(b) Upon activation, the B-lymphocyte undergoes clonal expansion (mitosis) and differentiation into an antibody-secreting plasma cell, increasing its rough endoplasmic reticulum (RER) and Golgi apparatus to synthesize large amounts of IgE. [2 Marks]

Context : 2 Study the the following diagram and answer the following questions.

Question 3: (a) Identify cells Q and R.

(b) Name event P and identify the primary chemical mediator released during this event.

Question 4: (a) Explain why the first exposure to the allergen does not cause the immediate release of chemical mediators at stage P.

(b) Describe what must happen at the cell surface during a secondary exposure to trigger event P.

Answer 3: (a) Cell Q: Helper T-cell (TH cell / CD4+ T-cell.

Cell R: Plasma Cell (or activated B-cell differentiating into Plasma Cell). [1 Mark]

(b) Event P: Degranulation (or Exocytosis of inflammatory granules). [1 Mark]

Mediator: Histamine (or Leukotrienes / Prostaglandins). [1 Mark]

Answer 4: (a) The initial exposure is the sensitisation phase where IgE antibodies are produced and bind to mast cells; no pre-formed IgE is present on mast cells yet to trigger mediator release. [1 Mark]

(b) The allergen must bind simultaneously to at least two adjacent IgE antibodies on the mast cell membrane, causing cross-linking of the Fc RI receptors, which triggers signal transduction leading to degranulation. [2 Marks]

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📝AO3 Experimental Skills & Data Interpretation (Graph & Table Questions)

Scientists tested the effectiveness of an anti-histamine drug (Drug X) on reducing vascular permeability (measured by dye leakage into skin tissue, measured in mg/cm2 in response to different allergen concentrations.

Allergen Conc. (μg/cm³)	Mean Dye Leakage without Drug X (mg/cm³)	Mean Dye Leakage with Drug X (mg/cm³)
0 (Control)	0.2 ± 0.05	0.2 ± 0.04
10	2.8 ± 0.31	0.8 ± 0.12
50	6.4 ± 0.55	1.9 ± 0.22
100	9.1 ± 0.82	5.2 ± 0.61

Question 3: (a) State the independent variable and the dependent variable in this experiment. [2 Marks]

(b) Evaluate the claim that "Drug X completely prevents allergic reactions caused by histamine." Use data from the table to support your response. [2 Marks]

Question 4: (a) Why was a control group 0 micro gram/cm3 allergen concentration) included in this experiment? [1 Mark]

(b) Explain what the standard deviation values indicate regarding the reliability of the data for 100 micro gram/cm3 allergen concentration compared to 10 micro gram/cm3. [2 Marks]

Answer 3: (a) Independent Variable: Allergen concentration (or presence/absence of Drug X). [1 Mark]

Dependent Variable: Mean dye leakage micro gram/cm3 vascular permeability. [1 Mark]

(b) Claim is unsupported/incorrect because: Dye leakage still increases in the presence of Drug X as allergen concentration increases (e.g., rises from 0.2 to 5.2 micro gram/cm3).

Evidence: Drug X significantly reduces vascular permeability compared to no drug (e.g., from 9.1 to 5.2 micro gram/cm3 at 100 micro gram/cm3), but does not completely prevent or reduce it to baseline control levels (0.2 micro gram/cm3. [2 Marks]

Answer 4: (a) Purpose of Control: To establish a baseline level of vascular permeability without allergen exposure and ensure that dye leakage is caused specifically by the allergen. [1 Mark]

(b) Standard Deviation Interpretation: The SD value is larger at 100 micro gram/cm3 (± 0.82 / ± 0.61) than at 10 micro gram/cm3 (± 0.31 / ± 0.12).

This indicates greater variability/spread of data around the mean at higher concentrations, making the mean value relatively less precise/reliable at higher allergen doses.

Study the following graphs and answer the following questions on the basis of observations.

Question 1 : Describe the trend in Histidine concentration (Graph A) between 1 hour and 11 hours after treatment.

Question 2 : Compare the time taken for Histidine (Graph A) and Histamine (Graph B) to reach their maximum concentration.

Question 3 : Histidine is converted into Histamine by the enzyme Histidine Decarboxylase. Using evidence from both graphs, explain why the peak of Histamine occurs after the peak of Histidine.

Question 4 : In Graph B, a shaded grey area is shown around the trend line. (a) What does this shaded area represent? (b) What can you conclude about the reliability of the data between 2 to 6 hours compared to 0 to 1 hour?

Question 5 : Calculate the rate of increase in Histidine concentration between 1 hour and 2 hours.

Answer 1 : Rapid increase: Histidine concentration rapidly increases from 1 hour to reach a peak at 2 hours (reaches 8-fold change). (1 mark)

Steep decline: Between 2 hours and 7 hours, histidine concentration sharply decreases from 8 to around 1.2-fold change. (1 mark)

Levelling off: From 7 hours to 11 hours, the concentration levels off / remains relatively constant near the baseline (around 1.0-fold change). (1 mark)

Answer 2 : 1. Histidine reaches its peak much faster, at 2 hours (at 8-fold change). (1 mark) 2. Histamine reaches its peak later, at 4 to 4.5 hours (at 1.5-fold change). (1 mark)

Answer 3 : 1. Histidine acts as the substrate for the conversion into histamine. (1 mark)

2. As histidine concentration rises sharply (peaking at 2 hours), it is gradually broken down / decarboxylated by enzymes to synthesize histamine. (1 mark)

3. Therefore, histamine accumulation takes time, resulting in a delayed peak (around 4 hours) as substrate levels drop. (1 mark)

Answer 4 : (a) Represents Standard Deviation / Confidence Interval / Range of variation in the data points. (1 mark)

(b) Between 2 to 6 hours, the shaded region is wider, indicating higher variability / lower precision / less reliability compared to 0–1 hour where variability is minimal. (1 mark)

Answer 5 :

Value at 1 hour = 1.0 fold change

Value at 2 hours = 8.0 fold change

Change in concentration= 8 - 1 = 7 fold

Time difference= 2 - 1 = 1 hour

Answer : 7.0 fold change per hour

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